That's only for OEM, retail can be removed from one machine and installed to another machine as many times as you want.
There are two main lines of thought on the op's question. Some who only use proper restore media from the manufacturer and those who use what "they have". Both groups though agree on the fact the client must have a legitimate COA.
Now if you are part of the second group, you could get by with six iso's. XP Home OEM, XP Pro OEM, universal Vista x86 and x64 and universal Win7 x86 and x64. "Theoretically" speaking of course, but it would cover most of what you will see on your bench.
Then add Driverpack Solutions and WSUS offline and you're good to go.
Now if the COA sticker is unreadable it means ordering restore media or buying new OS.
There is a much better process . . . You can completely slipstream all the latest updates, service packs, drivers and applications into a single ISO. For instance, You could get a Vista SP2 ISO and it will still take you several hours to track down all the drivers and install all the updates and restart the computer 50 times or you do all that same work once every few months or so, slipstream the ISO and have Vista installed on just about any machine in about 1.5 hrs. After that, you simply use the product key on the bottom of the laptops.
The ISO will be about 7gb-8gb or more depending on what you slipstream and you will probably have to have a 8gb-16gb USB drive to install it with, but trust me, its worth all the effort.
I literally have a Vista ISO (with all the latest updates, drivers and applications slipstreamed) that, once installed will:
1. look for an internet connection, if one is not found, then it will user drivers pack to install the necessary drivers.
2. If there is an internet connection, then it will use another utility to find all the latest drivers and install them. It will also run ninite to update all my slipstreamed applications and update my Kaspersky Definitions.
3. If there was not an internet connection and it had to install drivers from the drivers packs, it will do so, automatically restart the computer when done, look for an internet connection once again, and when it does it will start step two.
Basically, the only thing left to do is to install the 5 or 6 Windows updates left over from the last few weeks since that ISO was made.